Polish command palette hierarchy and footer

[brsbl]

Human comments

What was wrong

The approved command-palette hierarchy and interaction pass was implemented in a detached prototype commit rather than a registered stack layer. As a result, the existing stack's root-command screenshot correctly showed the old surface without a footer, while only Threads mode exposed keyboard guidance.

What changed

• Makes keyboard guidance persistent in both the root command list and Threads search mode.
• Clips and fades the scrollable result viewport at the fixed footer boundary.
• Applies the approved inset search treatment, Threads mode pill, recessed footer keycaps, terminal-versus-drill-in hierarchy, and quieter thread metadata.
• Restores query-match highlighting and empty-query Backspace navigation while preserving the later draft split-opening fix.
• Adds the focused production-backed palette story and regression coverage.

No host-daemon wire, public SDK, CLI, or persistence contract changes.

Screenshots

Exact current comparison: parent f6ec3892e99c59ae2e50412b34b6960efd626f9e → head 227283fa7436744da26c3637090cda621c65d7b3.

Exact current-head captures were produced from the branch web app at 1280×900 and are shown in the originating bb thread. They confirm that the footer is visible in both root-command and Threads modes. They are not yet embedded here because this session has no signed-in Browser surface for GitHub's attachment uploader. This PR is not screenshot-complete until the exact parent/head images are embedded.

How you verified

• Exact branch web app launched with scripts/bb-dev-app current.
• Chrome for Testing 151.0.7922.71 at 1280×900.
• Root mode: one visible footer with ↑↓ Select and ↵ Run; 12 command rows; Search commands… placeholder.
• Threads mode: one visible footer with ↑↓ Select, ↵ Open, ⌘↵ Split, and Backspace / Esc Back; Search title, project, or message… placeholder; Threads mode chip.
• Light, dark, and system themes: footer remains visible; the input stays transparent against the palette surface.
• Hard reload: footer remains present; no console errors observed.
• git diff --check.
• Remote CI: all applicable jobs passed in run 33455914812. Node Compatibility and Mobile E2E were skipped by workflow.

Fixes: N/A — focused command-palette polish layer.

BB-Thread-ID: thr_ccffp4w2p2

AGENT GENERATED

[brsbl]

Superseded by the standalone command-palette layer #2851 in the new approved stack.